Fix compiler warnings in 'sim'

[anachrocomputer]

intel.c: In function ‘read_byte’:
intel.c:27:23: warning: operation on ‘ptr’ may be undefined [-Wsequence-point]
27 | #define READC ((*ptr)++)
| ~~~~~~^~
intel.c:28:51: note: in expansion of macro ‘READC’
28 | #define GETB (xdigitconv(READC) * 16 + xdigitconv(READC))
| ^~~~~
intel.c:45:23: note: in expansion of macro ‘GETB’
45 | unsigned char val = GETB;
| ^~~~
and others.